The above.phone will work without a SIM card. You can connect to Wi-Fi networks to use any of the applications on the above.phone. Your Above Services Suite (email, chat, video conferencing, and search) should all work.
Using cellular calls and texts from your SIM card is insecure, these methods have lackluster encryption and can be intercepted by your cellular service provider or other entities.
Using your Above Services on WiFi gives you multiple layers of protection, encrypting your traffic and hiding it from outside parties.
If your phone service is registered under your name, know that using a SIM card will de-anonymize you as each SIM card is unique and broadcasts its identifier to your cellular service provider.